Bacterial community composition during two consecutive NE Monsoon periods in the Arabian Sea studied by denaturing gradient gel electrophoresis (DGGE) of rRNA genes

Horizontal and vertical variations in bacterial community composition were examined in samples collected during two Joint Global Ocean Flux Study (JGO FS) Arabian Sea cruises in 1995, The cruises, 11 months apart, took place d uring two consecutive NE Monsoon periods (January and December). Bacteria w ere harvested by filtration from samples collected in the mixed layer, mid- water, and deep sea at stations across the study area, Total bacterial comm unity genomic DNA was analyzed by PCR amplification of 16S rRNA gene fragme nts, followed by denaturing gradient gel electrophoresis (DGGE). In total, 20 DGGE bands reflecting unique or varying phylotypes were excised, cloned and sequenced. Amplicons were dominated by bacterial groups commonly found in oceanic waters (e.g,, the SAR11 cluster of alpha-Proteobacteria and cyan obacteria), but surprisingly none of the sequenced amplicons were related t o gamma-Proteobacteria or to members of the Cytophaga-Flavobacter-Bacteroid es phylum, Amplicons related to magnetotactic bacteria were found for the f irst time in pelagic oceanic waters. The DGGE banding patterns revealed a d ominance of approximate to 15 distinguishable amplicons in all samples. In the mixed layer the bacterial community was dominated by the same approxima te to 15 phylotypes at all stations, but unique phylotypes were found with increasing depth. Except for cyanobacteria, comparison of the bacterial com munity composition in surface waters from January and December 1995 showed only minor differences, despite significant differences in environmental pa rameters. These data suggest a horizontal homogeneity and some degree of se asonal predictability of bacterial community composition in the Arabian Sea . (C) 1999 Elsevier Science Ltd. All rights reserved.